1. Field of the Invention
The present invention is suitable for an image-pickup optical system, for example, for a digital camera, a video camera, a film-based camera, a TV camera, and the like, and for a projection optical system for a projector.
2. Description of the Related Art
An image-pickup optical system for a digital camera and a projection optical system for a liquid crystal projector are required to have a wide angle of view, a high zoom ratio, and a high optical performance in the entire zoom range. As a zoom lens which can easily widen an angle of view, a negative-lead type zoom lens is known which includes a first lens unit that is arranged closest to the object and has a negative refractive power.
As the angle of view is widened in the negative-lead type zoom lens, the secondary (spectrum of the) lateral chromatic aberration increases at the wide-angle end. As the zoom ratio is enlarged with widening the angle of view, the secondary spectra of the lateral chromatic aberrations increase both at the wide-angle end and the telephoto end. At this time, the secondary spectra of the lateral chromatic aberrations occur at the wide-angle end and the telephoto end in opposite directions, and it becomes difficult to correct them in a well-balanced manner.
In the conventional negative-lead type zoom lens, the chromatic aberration is corrected by using an optical material with a high dispersion and a high anomalous partial dispersion.
U.S. Pat. No. 7,480,102 B2 discloses a zoom lens in which a material with a high dispersion and a high anomalous partial dispersion is used respectively for a lens unit disposed on the object side of the aperture diaphragm, and for a movable lens unit on the image-side lens unit so as to reduce a variation of a chromatic aberration during the zooming.
Japanese Patent Laid-open No. 2008-158159 discloses a zoom lens in which a lens made of a material with a high dispersion and a high anomalous partial dispersion is used for a first lens unit on the object side so as to reduce the chromatic aberration.